The present invention relates generally to a method and apparatus for flotation separation of coal particles and similar materials, and more particularly pertains to an improved method and apparatus for beneficiating coal by flotation separation of a froth generated by a spiral, open flow spray nozzle such that ground coal particles may be separated from impurities associated therewith such as ash and sulfur.
Coal is an extremely valuable natural resource in the United States because of its relatively abundant supplies. It has been estimated that the United States has more energy available in the form of coal than in the combined natural resources of petroleum, natural gas, oil shale, and tar sands. Recent energy shortages, together with the availability of abundant coal reserves and the continuing uncertainties regarding the availability of crude oil, have made it imperative that improved methods be developed for converting coal into a more useful energy source.
Many known prior art processes for froth flotation separation of a slurry of particulate matter are based on constructions wherein air is introduced into the liquid slurry of particulate matter, as through a porous cell bottom or a hollow impeller shaft, thereby producing a surface froth. These prior art methods are relatively inefficient approaches, especially when large amounts of particulate matter are being processed. Generally, these techniques are inefficient in providing sufficient contact between the particulate matter and the frothing air. As a result, large amounts of energy were required to be expended to generate the froth. In addition, froth flotation techniques which permit bubbles to rise in the slurry can tend to trap and carry impurities such as ash in the froth slurry, and accordingly the resultant beneficiated particulate product frequently has more impurities therein than desired.
Methods have been suggested and are being explored in the beneficiation of coal, i.e., the cleaning of coal of impurities such as ash and sulfur, either prior to burning the coal or after its combustion. In one recently developed technique for beneficiation, termed herein chemical surface treating, raw coal is pulverized to a fine mesh size and is then chemically treated. According to this technique, the treated coal is then separated from ash and sulfur, and a beneficiated or cleaned coal product is recovered therefrom. In further detail, in the heretofore mentioned chemical surface treating process, coal is first cleaned of rock and the like, and is then pulverized to a fine size of about 48 to 300 mesh. The extended surfaces of the ground coal particles are then rendered hydrophobic and oleophilic by a polymerization reaction. The sulfur and mineral ash impurities present in the coal remain hydrophilic and are separated from the treated coal product in a water washing step. This step utilizes oil and water separation techniques, and the coal particles made hydrophobic can float in recovery on a water phase which contains hydrophilic impurities.
In greater detail, McGarry et al. U.S. Pat. No. 4,347,126 and Duttera et al. U.S. Pat. No. 4,347,127. both of which are commonly assigned herewith, disclose the flotation separation of coal particles from impurities associated therewith such as ash and sulfur. In these arrangements, a primary spray hollow jet nozzle is positioned above a flotation tank having a water bath therein, and sprays an input slurry through an aeration zone into the surface of the water. The spraying operation creates a froth on the water surface in which a substantial quantity of particular matter floats, while other components of the slurry sink into the water bath. A skimming arrangement skims the froth from the water surface as a cleaned or beneficiated product. A recycling operation is also provided wherein particulate materials which do not float after being sprayed through the primary spray nozzle are recycled to a further recycle, hollow jet spray nozzle to provide a second opportunity for recovery of the recycled particles.
One type of spray nozzle currently being used in a coal beneficiation process of the type described in these patents is a full jet nozzle, as is available commercially from Spraying Systems, Co., Wheaton, Ill. Several problems have arisen with this particular nozzle, design, including a recurring problem with clogging thereof. Tank covers, filter systems, larger nozzles and extreme care and frequent cleaning were necessary to alleviate this problem.
The full jet nozzle is characterized by a multiplicity of small apertures therein which results in the development of a substantial back pressure across each nozzle during its operation. Laboratory studies have demonstrated that this type of nozzle design creates too high of a back pressure in the system which resulted in wide discrepancies in test results thereof and reduced capacity. This type of hollow cone nozzle, with its high back pressure thereacross, is also subject to high wear because of its structural design.
The spiral, open flow type of nozzle contemplated for use in association with the present invention is available commercially from several different manufacturers in many different types of materials including polypropylene and tungsten carbides. The test results disclosed herein were run on spiral nozzles from Bete Fog Nozzle, Inc., Greenfield, Mass. Although nozzles of this type have been used commercially in various commercial enterprises, they have not been utilized in froth flotation separation or in a manner similar to that taught by the present invention.